Water Injection

You've finished tuning up your engine but you find that at full throttle the engine splutters or stalls. When you get the car on a diagnostic you find that the knock sensor is kicking in to protect the engine.

Engine knock is a condition where the fuel inside the engine ignites prematurely and occurs within an engine running at high compression. (It can also be caused by other factors including low octane/poor quality fuel or hot spots within the engine.)

For the purpose of this article we shall assume that the engine knock is occurring because your engine is highly tuned and running big compression figures or due to forced induction.

This can also be caused when adding or uprating the turbo in your engine. Your options are to rebuild the engine with lower compression pistons, reduce the airflow into the engine or use water injection.

Just as a fireman sprays water onto a fire to cool it down the water injection in an engine helps to quell ignition. It will literally slow up the burn of fuel and enable a more thorough combustion process. Some advocates of water injection list the cleaning effect this has on the engine and head but this is not a primary consideration.

Unless you are suffering from engine knock, water injection is pretty irrelevant but can literally save an enthusiastic tuning project.

The question is where to inject the water spray. Your main choices are after the compressor (turbo or supercharger or before it). Then we have options around injecting into each intake or just prior to the branch.

Your route depends a lot on the manifold and positioning of the compressor. Ideally each intake manifold header will have a water injection nozzle requiring a more complex injection controller. The simplest systems go just downstream of the compressor before the branch allowing the manifold flow to direct the water charge.

Your aim is to get the same amount of water to each cylinder, otherwise the engine will be out of dynamic balance and you will have to tune for the weakest common denominator.

Some choose to run their water injection pre turbo but others assume that this could have a detrimental effect on the long term life of the impellers. Proponents of this method will state that this allows the best atomisation of the water into the intake charge and also reduce the intake temperature allowing the turbo to work more efficiently (compressing air increases its temperature).

We would certainly agree that having a jet or large droplets of water hitting your impeller does not make sense.

The jet of water only really needs to start when you approach your critical knock threshold so there is little point injecting the water all of the time.

In an ideal world you need around 10 inches of intake length to mix the fuel/air and water in a uniform fashion.

Nozzle size is also vital and just like with a fuel injector you want a mist rather than a jet. A mist size of around 50 microns is acceptable with the droplets reducing in size as atomisation occurs. The smaller 1mm nozzle sizes are much more effective at producing a mist and allowing atomisation. The engine heat (and intake compression heat) will finish off the atomisation process.

How does one control the quantity of the water injected into the engine? The amount of water should vary according to engine load and rpm. As an example lets look at manifold pressures.

Manifold pressures can reach 11psi at 3500rpm and also hit the same 11psi at 7000rpm. As the engine speed is 100% greater you will also need to at least double the water injection. It really does pay to get a good quality water injection controller which takes the RPM and fuelling figures into the equation. Fuel delivery rate alone is a reasonably good indicator of the amount of water so a crude controller could take this into account.

Because water injection slows down the burn most common applications will benefit from slightly advanced timing of a few degrees. Because every engine is unique you need to carefully setup the advance. If you record little or no power gain then chances are that you already have the optimum timing. Advancing the timing further will just increase the cylinder pressure.

The downsides of water injection include corrosion of metal clips and joins within the intake. Rusty metal components going into the engine is a very bad idea. So check the intake for bare metal clips and replace these with corrosion resistant ones.

The next worst thing to happen is a sudden loss of water injection. This can be caused by injector failure, the tank running dry or some other component failure. A good water injection system will have a fail safe built in which reduces the engine power for example by reducing the boost pressure at the turbo. The more highly tuned the engine the bigger this problem is.

On a fast road tuned street car the knock sensor should be able to adjust the fuelling and it would be similar to using a low octane fuel. As long as you keep the revs low you will be able to use the car. When we are talking about a highly tuned drag machine the sudden loss of water and subsequent knock can be catastrophic and requires some serious thought. A backup system can be used or a twin injection system will provide a 50% injection if one fails.

Daily checks on your water injection system are prudent, and the more highly tuned the car the more vital this is. Always carry some spare water with you, check all delivery lines for chaffing or rubbing. Use filtered water and periodically check the intake for signs of corrosion.

Water injection typically has a similar effect on an engine to running higher octane fuel. As such it is irrelevant if you are not suffering from knock. However on NASP engines with turbos added it can make the difference between a drivable fun car and a lumpy dog of a car.

If you would like to discuss water injection applications for your car in more detail please contact us at JBS: office@jbsautodesigns.co.uk